1. Field of the Invention
The present invention relates to a flow rate detecting element and a flow rate sensor for measuring an amount of inlet air of an internal combustion engine, for example, and more particularly, to a flow rate detecting element and a flow rate sensor having a heating unit for measuring a flow velocity or a flow rate of a fluid, on the basis of the heat transfer phenomenon to the fluid, from the heating unit or a portion heated by the heating unit.
2. Description of the Related Art
FIG. 26 is a plan view illustrating a conventional thermosensitive flow rate detecting element disclosed, for example, in Japanese Unexamined Patent Publication No. 6-249,693; and FIG. 27 is a sectional view of FIG. 26 cut along the line XXVII--XXVII.
In FIGS. 26 and 27, insulating support films 23a and 23b are formed separately on a surface of a flat substrate 1 made of silicon. A heating resistance element 4 as a heating element is formed on the support film 23a. A fluid temperature measuring element 7 is formed on the support film 23b. Air spaces 27a and 27b are provided, respectively, on the flat substrate 1 under the heating resistance element 4 and the fluid temperature measuring element 7. These air spaces 27a and 27b are formed by applying etching from the other side of the flat substrate 1 so as not to damage the support films 23a and 23b and removing a part of the flat substrate 1 up to the support films 23a and 23b. Electrode terminals 28 are provided on one side of the flat substrate 1, and the heating resistance element 4 and the fluid temperature measuring element 7 are connected through a conductive duct 29 to electrode terminals 28.
A temperature control circuit shown in FIG. 25 comprises a Wheatstone bridge circuit for keeping temperature of the heating resistance element 4 at a certain temperature higher than a peripheral temperature detected by the fluid temperature measuring element 7. This Wheatstone bridge circuit has a side composed of the heating resistance element 4 and a resistance 21b, and another side composed of the fluid temperature measuring element 7 and a resistance 21a. A differential amplifier 22 acts to take balance of the bridge circuit by changing output potential to keep power consumed by the heating resistance element 4 at a certain level.
Control is performed so as to keep temperature of the heating resistance element 4 at a level higher by 200C than a peripheral temperature detected by the fluid temperature measuring element 7.
In the flow rate detecting element having the configuration as described above, the heating resistance element 4 is heated by current applied by the terminal 28 and the conductive duct 29. The heating resistance element 4 is designed so that resistance varies with temperature. The heating resistance element 4 is cooled by a flowing fluid. The extent of this cooling depends upon the mass flow of the fluid. The intensity of flow of the fluid is therefore determined by measuring electric resistance of the heating resistance element.
In the flow rate detecting element having the configuration as described above, heat generated in the heating resistance element 4 is conducted through the support films 23a and 23b and the flat substrate 1 to the fluid temperature measuring element 7. The fluid temperature measuring element 7 is therefore provided at a position free from heat affection from the heating resistance element 4.
A diaphragm structure is achieved by providing an air space 27a under the heating resistance element 4. A change in flow rate or flow velocity of the fluid to be measured can therefore rapidly be responded. Because the air space 27b is provided under the fluid temperature measuring element 7, it is possible to rapidly respond to a change in temperature of the fluid to be measured.
The conventional thermosensitive flow rate detecting element has a sensor unit of the diaphragm structure composed of a heating resistance element 4 to permit rapid response to a change in flow rate or flow velocity of the fluid to be measured. Further, the air space 27b reaching the support film 23b is provided on the flat substrate 1 to reduce the heat capacity of the fluid temperature measuring element 7 and to permit rapid response to a change in temperature of the fluid to be measured.
However, the conventional thermosensitive flow rate detecting element suffers from a problem of a serious decrease in strength of the flat substrate 1 because of the provision of the two air spaces 27a and 27b reaching the support films 23a and 23b on the flat substrate, and hence a decrease in reliability of the flow rate detecting element.
In the conventional flow rate detecting element, as described above, the two air spaces 27a and 27b reaching the support films 23a and 23b on the flat substrate 1 with a view to improving response to a change in flow rate or flow velocity of the fluid to be measured and response to a change in temperature of the fluid to be measured. It is therefore difficult to design a flow rate detecting element while ensuring a high reliability in terms of strength.
When the air space 27b under the fluid temperature measuring element 7 is abolished in order to ensure a satisfactory reliability in strength of the fluid temperature measuring element, the heat capacity of the fluid temperature measuring element 7 becomes larger, thus causing a decrease in response of the fluid temperature measuring element 7 to a change in fluid temperature. Since the heating resistance element 4 is controlled so as to be at a temperature higher by 200C than a temperature detected by the fluid temperature measuring element 7, a delay in response of the fluid temperature measuring element 7 to fluid temperature causes also a delay in temperature control of the heating resistance element 4, thus leading to a decrease in response of the flow rate sensor incorporating the flow rate detecting element.
The distance between the heating resistance element 4 and a fluid temperature measuring element 7 is short. Therefore, the fluid temperature measuring element 7 is thermally affected by the heating resistance element 4, heat generated in the heating resistance element 4 causes an increase in temperature of the fluid temperature measuring element 7. Temperature of the heating resistance element 4 would thus be controlled on the basis of the increased temperature of the fluid temperature measuring element 7, thus resulting in a thermal runaway of the heating resistance element 4. So as to be free from heat affection from the heating resistance element 4, therefore, it is necessary to provide the fluid temperature measuring element 7 at a prescribed distance from the heating resistance element 4, and this prevents downsizing effort of the apparatus.
When measuring an amount of inlet air of an automobile internal combustion engine, for example, it is necessary to detect an accurate temperature even during running, i.e., upon sudden change in inlet air temperature at entry or exit of a tunnel. The flow rate detecting element is therefore required to have a good response to an inlet air temperature. Further, because the maximum flow rate of inlet air sometimes reaches a value near 200 m/s, the flow rate detecting element is required to have a prescribed strength.
In the conventional flow rate detecting element, however, strength must be reduced for the improvement of response, and it is very difficult to achieve a design suitable for measuring an amount of inlet air of an internal combustion engine.